Embodiments of the present invention relate to the surface determination of the fluid level in the tubing-casing annulus of an oil well, gas well, or water well in a manner that does not require the speed of sound in the annulus to be known. The present invention provides for the accurate surface determination of the fluid level in the tubing-casing annulus through the imposition of a pressure wave of a known pulse width and shape.
Existing techniques that utilize pressure waves or acoustic waves to measure fluid level require the speed of sound in the casing gas to be known to make the fluid level determination. Under the generally utilized devices and methods, the round-trip time for the pressure pulse to return to the surface is measured and, assuming a value for the speed of sound in the casing gas, the distance to the fluid is calculated. In contrast, the present invention can determine the fluid depth without requiring any knowledge of the speed of sound in the casing gas.